1. Field of the Invention
The present invention relates to mounting substrates and mounting methods of electronic parts, and more specifically, to a mounting substrate and a mounting method of an electronic part wherein it is possible to prevent a void from remaining in a melt-capable connection member after an electronic part is connected.
2. Description of the Related Art
A high density electronic part such as BGA (Ball Grid Array) or CSP (Chip Size package), for example, a semiconductor device, may be flip chip mounted on a mounting substrate. See the Japan Laid-Open Patent Application No. 9-293961. That is, a semiconductor part such as the BGA or CSP has a solder ball functioning as an outside connection terminal. The solder ball is connected to a land formed on the mounting substrate so that the semiconductor part is mounted on the mounting substrate.
Recently, it has been strongly demanded that an electronic apparatus where the semiconductor device is installed, for example, a mobile terminal device, be made miniaturized and thin. Because of this, the semiconductor device is also made to have a high density so that it has been attempted to make the solder balls, functioning as the outside connection terminals, with a fine and have narrow pitch. More specifically, although the diameter of a conventional solder ball is 760 μm, the diameter of a recent solder ball is approximately 300 μm. Furthermore, the pitch between the conventional solder balls is 1270 μm, while the pitch between the recent solder balls is 500 μm.
FIG. 1 shows a method whereby an electronic part 5 such as a semiconductor device is flip chip mounted on a mounting substrate 1 in the related art. As shown in FIG. 2, a plurality of solder balls 8 is formed on a part substrate 6 of the electronic part 5. Corresponding to this, a plurality of substrate side lands 2 is formed on the mounting substrate 1. For the sake of convenience of explanation, a situation where the single substrate side land 2 is formed on the mounting substrate 1 and the single solder ball 8 is formed on the electronic part 5 is shown in FIG. 1, and explanation thereof is provided.
In order to mount the electronic part 5 on the mounting plate 1, the mounting plate 1 where the substrate side land 2 is formed as shown in FIG. 1-(A) is prepared. The solder paste is arranged on the mounting plate 1 by using a thick film printing method (screen printing method). The solder paste 3 is formed by mixing a powder state solder and flux. The solder paste 3 is in a uniform paste state.
More specifically, as shown in FIG. 1-(B), a screen for printing having a pattern 12 corresponding to a configuration of the solder paste 3 is arranged at an upper part of the mounting substrate 1. The solder paste 3 is installed into the pattern 12 by using a squeegee (not shown in FIG. 1). After that, the screen 11 for printing is removed from the mounting plate 1 and thereby, as shown in FIG. 1-(C), the solder paste 2 is arranged (applied) on the substrate side land 2.
Next, as shown in FIG. 1-(D) and FIG. 2, the solder ball 8 is positioned so as to face to the solder paste 3. The electronic part 5 moves down to the mounting plate 1 so that the solder ball 8 is mounted on and provisionally fixed to the solder paste 3. Next, the mounting plate 1 where the electronic part 5 is provisionally fixed is inserted into a reflow furnace so that a heating process is done. By the heating process, the powder state solder in the solder paste 3 and the solder ball 8 are made molten and the flux in the solder paste 3 is vaporized.
Because of this, the solder in the solder paste 3 and the solder ball 8 are unified. This unified solder is called a connection solder 9. As shown in FIG. 1-(E), the substrate side land 2 and the part side land 7 are connected by the connection solder 9. Therefore, the mounting substrate 1 and the electronic part 5 are electrically and mechanically connected.
As described above, in the case where the electric part 5 is mounted on the mounting substrate 1, the solder paste 3 is arranged on the substrate land 2 of the mounting substrate 1 in this related art. It is normal to use the screen printing method wherein the screen 11 for printing is used, as described above, to mount and arrange the solder paste 3 on the substrate side land 2. However, in this case, moisture and air may be mixed in the solder paste 3 at the time of screen printing. Furthermore, the moisture may be included in the flux included in the solder paste. The moisture or air is expanded at the time of heating such as the reflow process so that the volume of the moisture or air increases. The moisture or air whose volume is expanded is called a void.
However, since the amount of the moisture or air mixed in the solder paste 8 or the flux is very small, the void may not be a problem in the conventional art wherein making the solder ball fine and making the pitch between the solder balls narrow are not so greatly required and the volume of the solder ball (the connection solder 9) is large. However, since the ball 8 is being made fine recently and continuing, the ratio of the volume of the void 10 to the volume of the connection solder 9 may relatively increase.
FIG. 3 shows an example where the ratio of the volume of the void 10 to the volume of the connection solder 9 increases. In the example shown in FIG. 3, the void 10 in the connection solder 9 is made big, so that an area where the connection solder 9 and the substrate side land 2 is connected is only an area shown by an arrow A in FIG. 3. If the volume of the void 10 occupied in the connection solder 9 increases, an electric characteristic and the mechanical connection strength of the solder paste 9 and the substrate side land 2 (or the part side land 7) is degraded. In a worst case, a problem of solder poor connection (open failure) may happen so that reliability of the solder connection may be degraded.
An X-ray may be used for viewing the electronic part 5 connected to the mounting substrate 1 from the plane, as a method for detecting a generation of the void 10. However, it is difficult to view the void 10 by X-ray imaging. Furthermore, since the void 10 is situated at a lower part of the part side land 7 made of metal, the part side land 7 is an obstacle for the X-ray imaging and therefore it is further difficult to detect the void 10.
In addition, recently and continuing, a solder having no lead (lead free solder) is frequently used as one of measures to alleviate an environmental problem. More specifically, Sn—Ag—Cu solder (melting point: 217° C.) which has a melting point higher than a melting point of a conventional Sn—Pb eutectic solder (melting point: 183° C.) is frequently used for the solder ball 8.
On the other hand, due to a heat-resisting problem of the electronic part 5, it is difficult to make a soldering temperature profile have a high temperature. It is normal to make the soldering temperature profile have a peak temperature which is 10-20° C. higher than the melting point. (The conventional Sn—Pb eutectic solder can maintain 30° C. higher than the melting point.) Making the soldering temperature profile have a high temperature, changing from the flux in the solder paste to a solvent having a high boiling point, and others, cause a delay of the flow of solder at the time of solder connection.
Because of this, the amount of the generated void 10 discharged to outside of the connection solder 9 is made small so that the amount of the void 10 remaining in the connection solder 9 increases. As a result of this, the volume of the void 10 in the connection solder 9 increases and therefore the reliability of the solder connection may be degraded.